Abstract

The first HiRISE images of Elysium Planitia (Mars) show areas both near the centreand the edges of the 800 x 200 km area of fractured, drifted and rotated rafts up totens of kilometers across, which have been interpreted as a dustcovered frozen sea a few million years old. Image TRA 000854 1855 is situated near the centre of the sea, encompassing a junction between an area of relatively static ice to the north, and a channel in which rafts have drifted to the west.

These images have a resolution of about 0.3m, and at this scale the large (100m to 3 km) fractured rafts can be seen to have a 10% cover of platy blocks 1m to 6m across, interspersed with a honeycombed pattern of networked aeolian ridges, with cells about 3m wide. The blocky appearance is very similar in metre-scale morphology to broken and re-frozen arctic pack-ice. Between the rafts, particularly in the northern part of the image, is smoother terrain of lower albedo, topographically lower (from shadows) than the rafts at contacts, with prominent patterned ground comprising centrally-domed polygons 5m-10m wide, similar to polygonally-cracked ground on subliming ice in Antarctica. In places these polygons are aligned almost parallel and/or normal to the raft edges, and there are patterns of more pronounced valleys spaced about 30m apart, running longitudinally down each lane. We interpret this terrain as subliming sea ice, now stabilized by surface dust and a sublimation till. A third type of terrain is found only in the southern part of the image, where the pale, smooth lanes between rafts are aligned more east-west. There are still traces of polygons, but these are much fainter and wider (20m - 40m). This terrain much more heavily cratered than the previous two terrain units, and size-frequency crater distributions show that it is about an order of magnitude older. We believe it to represent the sea bed after the sea ice above had sublimed away.